Digital data is often stored by being electronically “written” to the surface of a disk in a hard-disk drive. This digital data or information is retrieved, or read back, by means of a magneto-resistive (“MR”) element in close proximity to the spinning disk. The small signal (in the millivolt range) from the MR element in a hard disk drive must be AC-coupled before it is presented to the reader amplifier due to the significant DC voltage (up to several hundred millivolts) across the MR element. The low-corner −3 dB frequency of that AC coupling must be set low enough so that none of the significant bandwidth of the signal is lost. Low-corner −3 dB frequencies on the order of 1 MHz and above are conventionally available. However, recent advances in disk drive technology such as vertical recording, have required that the low-corner −3 dB frequency be lowered by around an order of magnitude. This is significant enough to require modifications to the design of the reader amplifier front-end. The reader amplifier input stage has traditionally been comprised of NPN bipolar transistors because of their low noise characteristics. However, the base current associated with these transistors makes it difficult to achieve the low-corner −3 dB frequency required for vertical recording.
The transconductance of an all-bipolar junction transistor pair can be set by a predetermined tail current and conventional arrangements use NPN transistors as the input differential pairs. As such, the gain can be accurately determined from the tail current as bipolar transconductance is set precisely by the current. However, the transconductance of an NPN/NMOS or PNP/PMOS differential pair is not set by a predetermined tail current as is the case with an all-bipolar pair.
The present invention utilizes a scaled master differential stage with a given offset to force a given output current, preceded by a balanced stage circuit comprised of the bipolar/MOS differential pair to get a balanced offset. The result is a tail current for the main amplifier input stage.